Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Net sales	$ 331.9	$ 320.6	$ 334.9	$ 343.2	$ 316.6	$ 336.4	$ 367.2	$ 371.7	$ 1,330.6	$ 1,391.9	$ 1,356.1
Cost of products sold, excluding depreciation and amortization and other items	252.4	254.7	250.4	262.0	251.0	267.3	294.8	302.3	1,019.5	1,115.4	1,117.5
Lower of cost or market inventory write-down	0.0	0.0	0.0	4.9	2.6	0.0	0.0	0.0	4.9	2.6	0.0
Unrealized (gain) loss on derivative instruments	(1.8)	(2.0)	(10.9)	(4.0)	(4.3)	1.7	1.5	4.5	(18.7)	3.4	10.4
Gross Profit	81.3	67.9	95.4	80.3	67.3	67.4	70.9	64.9
Operating income	45.3	29.8	57.9	44.8	35.2	40.5	37.0	(458.6)	177.8	(345.9)	137.9
Net income	$ 24.5	$ 14.9	$ 26.0	$ 26.3	$ 13.3	$ 22.1	$ 20.2	$ (292.2)	$ 91.7	$ (236.6)	$ 71.8
Net income (loss) per common share:
Basic (in dollars per share)	$ 1.39	$ 0.84	$ 1.45	$ 1.47	$ 0.76	$ 1.29	$ 1.19	$ (16.85)	$ 5.15	$ (13.76)	$ 4.02
Net Income (loss) per common share, Diluted:
Diluted (in dollars per share)	$ 1.37	$ 0.82	$ 1.43	$ 1.44	$ 0.73	$ 1.21	$ 1.11	$ (16.85)	$ 5.09	$ (13.76)	$ 3.86